As an electrically inducible, i.e. polar or dipoles containing liquid, mostly demineralized (DI) water is used. DI water is readily available and has good dipole properties. Furthermore, it does not mix with oil as an electrically inert fluid, and for this reason, virtually all devices that have been known so far and which work with electrowetting use combinations of water and oil.
For example, a first application of a liquid-operated field effect transistor (D. Y. Kim and A. J. Steckl: “Liquid-state field-effect transistors using electrowetting”; Applied Physics Letters, Vol. 90, No. 4; Jan. 22, 2007) uses a combination of water, which has been supplemented with sodium chloride for conductivity reasons, and a non-polar, not further described, oil.
In a second application, a fluidic display, in which the pixels are operated using electrowetting, water and oil are used as well (Johan Feenstra & Rob Hayes: “Liquavista electrowetting displays”, Liquavista BV, The Netherlands, January 2006; http://www.liquavista.com/files/LQV060828XYR-15.pdf; see e.g. also U.S. Pat. No. 7,304,786, “Methods and apparatus for bi-stable actuation of displays”).
In a third application relating to a lens consisting of liquid, a water and oil combination also is used, with both liquids having similar densities, but different refractive indexes. When a voltage is applied to an electrode, the interface between both liquids alters its form resulting in the lens effect (www.varioptic.com). Furthermore, the patent associated with this technology (WO/1999/018456 LENS WITH VARIABLE FOCUS) mentions, only vaguely, a first, conductive and a second isolating liquid, but specifically discloses for the first liquid solely water or “all organic or non-organic liquids that are conductive or may be made conductive”.
In a fourth application, relating to the so-called “lab on a chip” systems, again water is used embedded in oil. By applying several electric fields, liquid drops may be moved, separated from each other and connected with each other (Y. Fouillet et al.: “EWOD Digital Microfluidics For Lab on A Chip”, Keynote Paper, Proceedings of ICNMM 2006, June 2006, Limerick, Ireland).
Whereas the advantages of the ready and secure as well as cheap availability of water as the electrically inducible liquid are obvious, the limited temperature range has turned out to be problematic. As water is known to freeze at 0° C. and to boil at 100° C., all applications based on electrowetting and using water as liquid are restricted to this temperature range. In addition to the mere failure of an electrowetting device based on water which is to be expected outside the allowed temperature range, it must be assumed that it will be destroyed when leaving the allowed temperature range, because water has the property of expanding in the solid state of freezing as well as in the gaseous state of boiling, accordingly requiring more volume. Whereas in the case of overheating, appropriate extra reservoirs may be used providing the necessary larger volume, supercooling is more problematic, as the frozen liquid does not move anymore, so that a permanent damage of the fluidic parts of the system can hardly be excluded.
Thus, conventionally constructed devices may not be used at low temperatures, such as e.g. outside in winter, or at high temperatures, such as e.g. as display devices in the area of combustion machines, or e.g. in bright sun light on or at the dashboard of an automobile.